1. Technical Field
The present invention relates to a tsunami breakwater wall of multilayered steel pipe pile structure and its construction method. In particular, the invention relates to a tsunami breakwater wall of multilayered steel pipe pile structure and its construction method that is advantageous in withstanding tsunamis and earthquakes and effective for providing measures against tsunamis at areas such as seashores and seaports.
2. Related Art
The off the Pacific coast of Tohoku earthquake (the huge earthquake that occurred with the epicenter off the Sanriku Pacific Coast on Mar. 11, 2011) had brought damages due to the tsunami creating the need for immediate provision of tsunami-resistant facilities such as a tsunami breakwater and the like at the seashore, seaports and the like. Various facilities have already been built in these areas and are used for navigation of ships, cargo work, freight transport complex and the like and therefore, tsunami-resistant facilities such as tsunami breakwaters need to be built taking such circumstances into consideration.
Japanese Utility Model Application Examined Publication No. 6-40027 (PTL 1) describes an example of a breakwater. This breakwater is constructed by placing into the ground in a row, a plurality of steel pipes each of them being one to two meters in diameter, in a wall form such that a gap is formed between adjacent steel pipes and coupling between the top parts of the row of steel pipes with concrete copings.
Two openings one above and one under a portion proximate the water level, are provided to the surfaces of the above described steel pipes. And a horizontally facing partition panel is provided to each of the portions within the steel pipes corresponding to the middle of the two openings.
According to the breakwater described in PTL 1, waves passing through the gaps between the steel pipes can be reduced by dissipating the waves close to the water level with the openings and partition panels in the steel pipes of the row of steel pipes thus improving the wave-dissipating capability.
Additionally, Japanese Utility Model Application Examined Publication No. 7-23022 (PTL 2) describes another example of a breakwater. This breakwater is constructed as a gravity-type levee by installing a plurality of steel foundation piles into the ground of the seabed, coupling between the top parts of the foundation piles with reinforced concrete footings to structure an artificial ground and placing a concrete caisson above this artificial ground.
An upright portion is provided to the side end part of the above described footing, a mound layer composed of waste rock or gravel is formed to a portion surrounded by this upright portion and the top face of the footing, and a caisson is set on top of this mound layer. Further, the top portions of the foundation piles are inserted into the outer tubular steel pipes embedded in the footing and concrete is placed in the gaps between the foundation piles and the outer tubular steel pipes in the aforementioned state so to couple the top portions of the foundation piles to the footing.
According to the breakwater described in PTL 2, the constituent material of the mound layer between the artificial ground and the gravity-type levee is prevented from scattering during construction and use, so the ground contact pressure of the gravity-type levee can be transferred to the footing of the artificial ground in a dispersed and eased manner so that the load on the footing can be alleviated. Furthermore, since the structure is such that the artificial ground and the gravity-type levee are not fixed, horizontal force applied to the gravity-type levee is transferred to the artificial ground by friction, the horizontal force that is equal to or greater than the frictional force is not transferred to the foundation piles of the artificial ground even when a wave pressure beyond assumption acts on the gravity-type levee thus alleviating the load on the foundation piles. Additionally, since the top portions of foundation piles are inserted into the outer tubular steel pipe of the footing, the spaces between the foundation piles and the outer tubular steel and pipes are coupled with concrete, the coupling portions between the footing of the foundation piles can be reinforced.
Japanese Patent Application Laid-open Publication No. 2001-3331 (PTL 3) describes further another example of a breakwater. This breakwater includes a plurality of piles installed in two rows so that the head portions protrude from the surface of the seabed, a curtain wall that is integrally provided to the row of piles on the off shore side and orthogonal to the direction in which the waves move, and a connecting block that is provided to integrate the top portions of the four piles.
The aforementioned curtain wall is formed by combining a plurality of approximately T-shape wave-dissipating blocks. Further, each of the wave-dissipating blocks has provided thereto a penetration hole that vertically penetrates each wave-dissipating block. And a wave-dissipating block and a pile are integrated by inserting the pile into this penetration hole.
According to the breakwater described in PTL 3, a curtain wall composed of a plurality of wave-dissipating blocks supported by a row of piles on the off shore side can be constructed by inserting each of the piles of the row of piles on the off shore side into the penetration holes of each of the wave-dissipating blocks. Additionally, the row of piles on the off shore side and the row of piles on the shore side can be integrated by integrating with a connection block between the top portions of four piles of the row of piles on the off shore side and the row of piles on the shore side. Accordingly, the construction performance can be improved and thus allowing the construction costs to be reduced.